Reference is made to commonly-assigned copending U.S. patent application Ser. No. 09/742,525, filed of even date herewith, entitled xe2x80x9cProcess for Laminating Ink Jet Printxe2x80x9d, of Romano, Jr., et al; and U.S. patent application Ser. No. 09/742,752, filed of even date herewith, entitled xe2x80x9cInk Jet Recording Elementxe2x80x9d, of Romano, Jr., et al.
This invention relates to an ink jet printing process for improving the durability of an ink jet image.
Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods which may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous ink jet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets are projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various ink jet printers can be classified as either dye-based or pigment-based. A dye is a colorant which is molecularly dispersed or solvated by a carrier medium. The carrier medium can be a liquid or a solid at room temperature. A commonly used carrier medium is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier medium. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor light-fastness. When water is used as the carrier medium, such inks also generally suffer from poor water-fastness.
U.S. Pat. No. 6,087,051 relates to an ink jet recording element containing a protective overcoat layer of an aqueous polyurethane resin or an aqueous polyacryl resin. In addition, there are comparison examples in that patent which use a polyester resin, and the aqueous polyurethane resin and polyacryl resins are said to have advantages over the polyester resin. However, there are problems using a polyurethane resin or an aqueous polyacryl resin in that these resins have to be synthesized from virgin raw materials and the resins cannot be recycled.
It is an object of this invention to provide an ink jet printing process for improving the durability of an ink jet image. It is another object of this invention to provide an ink jet printing process wherein an overcoat layer is applied to an ink jet image with a material that can be made from recycled materials and is capable of being recycled.
These and other objects are achieved in accordance with the present invention which comprises an ink jet printing process for improving the durability of an ink jet image comprising:
a) providing an ink jet recording element comprising a support having thereon an image-receiving layer containing an ink jet image; and
b) applying over the surface of the image-receiving layer an overcoat layer of a water-dispersible, hydrophobic polyester resin having the following general formula:
In-P-Am 
wherein
I is an ionic group;
n is an integer from 1-3;
P is a polyester backbone;
A is an aliphatic group comprising a straight or branched chain fatty acid or triglyceride thereof having from about 6 to about 24 carbon atoms; and
m is an integer from 3-8.
It was found that the durability of an ink jet image is improved using an overcoat layer of a material that is capable of being recycled.
The ionic groups I in the above formula which provide the polymer with water-dispersibility are typically derived from a carboxylic acid group which is introduced into the resin by polyacid monomers such as trimellitic anhydride, trimellitic acid, or maleic anhydride or sulfonate groups which come from monomers such as dimethyl 5-sulfoisophthalate, dimethyl 5-sulfo, 1,3-benzenedicarboxylate, sulfoisophthalate ethylene glycol, dihydroxyethyl-5-sulfo 1,3-benzenedicarboxylate, or from sulfonated alkenically unsaturated end groups as described in U.S. Pat. No. 5,281,630, the disclosure of which is hereby incorporated by reference. The weight percent of ionic monomers in the resin is from 1% to 20%, but 1% to 10% is preferred.
The backbone P of the polymer in the above formula is composed of polyester groups. It can be any linear or branched polyester made using polyacids and polyalcohols. The weight percent of the polyester backbone ingredients range from 30-80% of the whole resin, with the most preferred being 50-60% by weight. Examples of aromatic dicarboxylic acids useful in the backbone polyester polymer, P, employed in the invention include, but are not limited to, terephthalic, isophthalic, phthalic, and 2,6-naphthoic, succinic, glutaric, adipic, 1,4-cyclohexane dicarboxylic, maleic, fumaric, and azelaic. The polyalcohol component of the polyester can be virtually any dihydroxy functional compound. Aliphatic and alicyclic glycols would be the most useful. Useful glycols include, but are not limited to, ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol, dipropylene glycol, tripropylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, cyclohexanedimethanol, diethylene glycol, and triethylene glycol.
The backbone polyester consisting of any combination of the above polyacids and glycols may further directly include or incorporate by transesterification a multifunctional polyol selected from, but not limited to, glycerol, trimethylolpropane, erythritol, pentaerythritol, trimethylolethane, or a monosaccharide.
As noted above, A in the above formula is an aliphatic group comprising a straight or branched chain fatty acid or triglyceride thereof having from about 6 to about 24 carbon atoms, such as stearic, oleic, palmitic, lauric, linoleic, linolenic, behenic acid, or their mixtures. These can come from hydrogenated or unhydrogenated animal or vegetable oil, such as beef tallow, lard, corn oil, or soy bean oil. The weight percent of the aliphatic moiety can be 10-60%, with 20-40% by weight being the preferred amount.
In a preferred embodiment of the invention, the water-dispersible, hydrophobic polyester resin employed comprises a reaction product of 30-70% by weight of a poly(ethylene terephthalate) condensation polymer; 5-40% by weight of a hydroxy functional compound having at least two hydroxyl groups; 1-20% by weight of a carboxy functional compound having at least two carboxyl groups and 10-60% by weight of a C6-C24 straight chain or branched fatty acid or triglyceride. The resin is further characterized in that the hydroxy functional compound is present at 1-3 times the equivalents of the hydrophobic moiety. The preparation of such hydrophobic polyester resins is described in detail in U.S. Pat. No. 5,958,601, the disclosure of which is hereby incorporated by reference. In another preferred embodiment, the water-dispersible, hydrophobic polyester resin comprises water-dispersed transesterified polyester, e.g., poly(ethylene terephthalate) transesterified in the presence of stearic acid and trimellitic acid, or oleic acid and trimellitic acid.
In another preferred embodiment of the invention, the water-dispersible, hydrophobic polyester as described above is physically mixed with a thermoplastic or thermosetting polymer. The thermoplastic or thermosetting polymer lends added hydrophobicity to the layer, as well as enhanced coating flexibility and serves as a diluent to the polyester component to minimize cross-linking which would deleteriously alter coating properties.
Examples of such thermoplastic or thermosetting polymers useful in the invention include, but are not limited to, carboxylated styrene butadiene, styrene/acrylate or methacrylate ester compositions containing acrylic or methacrylic acids, hydrolyzed styrene maleic anhydride copolymers, styrene maleic acid salt copolymers, styrene maleic ester copolymers, styrene (meth)acrylate copolymers, styrene (meth)acrylate ester copolymers, styrene acrylate ester acrylonitrile terpolymers, acrylonitrile (meth)acrylate salt copolymers, polycarbonate-based polyurethanes, polyester-based polyurethanes, cellulose polymers, such as methyl cellulose and cellulose acetate butyrate, polyesters, polyamides, polyacetals, epoxy polymers, phenoxy polymers, etc. In a preferred embodiment, the water-dispersible, hydrophobic polyester resin and thermoplastic or thermosetting polymer is present in a ratio from about 1:4 to 4:1.
A preferred aqueous dispersion of a mixture of carboxylated styrene butadiene copolymer and a hydrophobic polyester of the composition generally described above is commercially available as EvCote(copyright) PWRHS-37 from EvCo Research Incorporated, Atlanta, Ga., USA.
In another preferred embodiment, the overcoat layer may also contain a synthetic or natural wax, such as an aqueous dispersion of high density polyethylene, Jon Wax 26(copyright) (S. C. Johnson Co.) or an aqueous dispersion of carnauba wax (Michelman Co.); and/or a microgel, such as a microgel of methyl methacrylate/ethylene glycol dimethacrylate/acrylic acid.
Ink jet inks used to prepare the images to be protected using the invention are well-known in the art. The ink compositions used in ink jet printing typically are liquid compositions comprising a solvent or carrier liquid, dyes or pigments, humectants, organic solvents, detergents, thickeners, preservatives, and the like. The solvent or carrier liquid can be solely water or can be water mixed with other water-miscible solvents such as polyhydric alcohols. Inks in which organic materials such as polyhydric alcohols are the predominant carrier or solvent liquid may also be used. Particularly useful are mixed solvents of water and polyhydric alcohols. The dyes used in such compositions are typically water-soluble direct or acid type dyes. Such liquid compositions have been described extensively in the prior art including, for example, U.S. Pat. Nos. 4,381,946; 4,239,543 and 4,781,758, the disclosures of which are hereby incorporated by reference.
Any known ink jet image-receiving layer can be used in the present invention. For example, the image-receiving layer may consist primarily of inorganic oxide particles such as silicas, modified silicas, clays, aluminas, fusible beads such as beads comprised of thermoplastic or thermosetting polymers, non-fusible organic beads, or hydrophilic polymers such as naturally-occurring hydrophilic colloids and gums such as gelatin, albumin, guar, xantham, acacia, chitosan, starches and their derivatives, and the like; derivatives of natural polymers such as functionalized proteins, functionalized gums and starches, and cellulose ethers and their derivatives; and synthetic polymers such as polyvinyloxazoline, polyvinylmethyloxazoline, polyoxides, polyethers, poly(ethylene imine), poly(acrylic acid), poly(methacrylic acid), n-vinyl amides including polyacrylamide and polyvinylpyrrolidone, and poly(vinyl alcohol), its derivatives and copolymers; and combinations of these materials. Hydrophilic polymers, inorganic oxide particles, and organic beads may be present in one or more layers on the substrate and in various combinations within a layer.
A porous structure may be introduced into image-receiving layers comprised of hydrophilic polymers by the addition of ceramic or hard polymeric particulates, by foaming or blowing during coating, or by inducing phase separation in the layer through introduction of nonsolvent.
In practice, various additives may be employed in the image-receiving layer and overcoat. These additives include surface active agents surfactant(s) to improve coatability and to adjust the surface tension of the dried coating, acid or base to control the pH, antistatic agents, suspending agents, antioxidants, hardening agents to cross-link the coating, antioxidants, UV stabilizers, light stabilizers, and the like. In addition, a mordant may be added in small quantities (2%-10% by weight of the base layer) to improve waterfastness. Useful mordants are disclosed in U.S. Pat. No. 5,474,843.
The layers described above, including the image-receiving layer and the overcoat layer, may be coated by conventional coating means onto a transparent or opaque support material commonly used in this art. Coating methods may include, but are not limited to, wound wire rod coating, slot coating, slide hopper coating, gravure, curtain coating, and the like. Some of these methods allow for simultaneous coatings of both layers, which is preferred from a manufacturing economic perspective.
The image-receiving layer may be present in any amount which is effective for the intended purpose. In general, good results are obtained when it is present in an amount of from about 5 to about 30 g/m2, preferably from about 8 to about 15 g/m2, which corresponds to a dry thickness of about 5 to about 30 xcexcm, preferably about 8 to about 15 xcexcm.
The overcoat layer may be applied to the ink jet image in accordance with the invention either through a separate thermal or piezoelectric printhead, or by any other method which would apply the material evenly to the image, such as a spray bar. Methods of applying a overcoat layer are disclosed in commonly-owned U.S. patent application Ser. No. 09/083,673 filed May 22, 1998, entitled xe2x80x9cPrinting Apparatus With Spray Bar For Improved Durabilityxe2x80x9d of Wen et al. and U.S. patent application Ser. No. 09/083,876, filed May 22, 1998, entitled xe2x80x9cInk Jet Printing Apparatus With Print Head For Improved Image Qualityxe2x80x9d of Wen et al., the disclosures of which are incorporated herein by reference. Other methods for applying the overcoat layer include submerging the element into a tank containing a liquid dispersion of the polyester or by extrusion of the polyester on top of the recording element.
In general, the overcoat layer may be present at a dry thickness of from about 0.1 to about 5 xcexcm, preferably from about 0.25 to about 3 xcexcm.
The support for the ink jet recording element used in the invention can be any of those usually used for ink jet receivers, such as paper, resin-coated paper, poly(ethylene terephthalate), poly(ethylene naphthalate) and microporous materials such as poly polyethylene polymer-containing material sold by PPG Industries, Inc., Pittsburgh, Pa. under the trade name of Teslin(copyright), Tyvek(copyright) synthetic paper (DuPont Corp.), and OPPalyte(copyright) films (Mobil Chemical Co.) and other composite films listed in U.S. Pat. No. 5,244,861.
The support used in the invention may have a thickness of from about 50 to about 500 xcexcm, preferably from about 75 to 300 xcexcm. Antioxidants, antistatic agents, plasticizers and other known additives may be incorporated into the support, if desired. In a preferred embodiment, either paper or poly(ethylene terephthalate) is employed.
In order to improve the adhesion of the image-receiving layer to the support, the surface of the support may be subjected to a corona-discharge-treatment prior to applying the image-receiving layer.
In addition, a subbing layer, such as a layer formed from a halogenated phenol or a partially hydrolyzed vinyl chloride-vinyl acetate copolymer can be applied to the surface of the support to increase adhesion of the image-receiving layer. If a subbing layer is used, it should have a thickness (i.e., a dry coat thickness) of less than about 2 xcexcm.
The following examples are provided to illustrate the invention.